A process in which information is embedded in material for the purpose of identifying the material is referred to as watermarking.
Identification code words are applied to versions of material items for the purpose of identifying the version of the material item. Watermarking can provide, therefore, a facility for identifying a recipient of a particular version of the material. As such, if the material is copied or used in a way which is inconsistent with the wishes of the distributor of the material, the distributor can identify the material version from the identification code word and take appropriate action.
Co-pending UK patent applications with serial numbers 0129840.5, 0129836.3, 0129865.2, 0129907.2 and 0129841.3 provide a practical watermarking scheme in which a plurality of copies of material items are marked with a digital watermark formed from a code word having a predetermined number of coefficients. The watermarked material item is for example an image. In one example, the apparatus for introducing the watermark transforms the image into the Discrete Cosine Transform (DCT) domain. The digital watermark is formed from a set of randomly distributed coefficients having a normal distribution. In the DCT domain each code word coefficient is added to a corresponding one of the DCT coefficients. The watermarked image is formed by performing an inverse DCT.
Any watermarking scheme should be arranged to make it difficult for users receiving copies of the same material to collude successfully to alter or remove an embedded code word. A watermarking scheme should therefore with high probability identify a marked material item, which has been the subject of a collusion attack. This is achieved by identifying a code word recovered from the offending material. Conversely, there should be a low probability of not detecting a code word when a code word is present (false negative probability). In addition the probability of falsely detecting a user as guilty, when this user is not guilty, should be as low as possible (false positive probability).